Production of ethanol by microorganisms provides an alternative energy source to fossil fuels and is therefore an important area of current research. It is desirable that microorganisms producing ethanol, as well as other useful products, be capable of using xylose as a carbon source since xylose is the major pentose in hydrolyzed lignocellulosic biomass. Biomass can provide an abundantly available, low cost carbon substrate. Zymomonas mobilis and other bacterial ethanologens which do not naturally utilize xylose have been genetically engineered for xylose utilization by introduction of genes encoding 1) xylose isomerase, which catalyses the conversion of xylose to xylulose; 2) xylulokinase, which phosphorylates xylulose to form xylulose 5-phosphate; 3) transketolase; and 4) transaldolase.
There has been success in engineering Z. mobilis strains for xylose metabolism (U.S. Pat. No. 5,514,583, U.S. Pat. No. 5,712,133, U.S. Pat. No. 6,566,107, WO 95/28476, Feldmann et al. (1992) Appl Microbiol Biotechnol 38: 354-361, Zhang et al. (1995) Science 267:240-243), as well as a Zymobacter palmae strain (Yanase et al. (2007) Appl. Environ. Mirobiol. 73:2592-2599). However, typically the engineered strains do not grow and produce ethanol as well on xylose as on glucose. Strains engineered for xylose utilization have been adapted by serial passage on xylose medium, resulting in strains with improved xylose utilization as described in U.S. Pat. No. 7,223,575 and commonly owned and co-pending U.S. Pat. No. 7,741,119. Disclosed in commonly owned and co-pending US Patent App. No. US 2009-0246846 A1 is the finding that an adapted strain with higher xylose utilization has increased xylose isomerase activity, and engineering for improved xylose utilization by expression of xylose isomerase from a mutated, highly active Zymomonas mobilis glyceraldehyde-3-phosphate dehydrogenase gene promoter (Pgap). However xylose utilization is still not comparable to glucose utilization.
There remains a need for strains of Zymomonas, and other bacterial ethanolagens, which have further improvement in xylose utilization.